Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU (v3)

Paul E. McKenney
2009-04-18T14:15:20Z

On Sat, Apr 18, 2009 at 01:40:01PM +0400, Evgeniy Polyakov wrote:
> Hi.
>
> On Thu, Apr 16, 2009 at 06:28:12PM -0700, Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > +/* Single bit for grace-period index, low-order bits are nesting counter. */
> > +#define RCU_FGP_COUNT 1UL
> > +#define RCU_FGP_PARITY (1UL << (sizeof(long) << 2))
> > +#define RCU_FGP_NEST_MASK (RCU_FGP_PARITY - 1)
> > +
> > +extern long rcu_fgp_ctr;
> > +DECLARE_PER_CPU(long, rcu_fgp_active_readers);
> > +
> > +static inline void rcu_read_lock_fgp(void)
> > +{
> > + long tmp;
> > + long *uarp;
> > +
> > + preempt_disable();
> > + uarp = &__get_cpu_var(rcu_fgp_active_readers);
> > + tmp = *uarp;
> > + if (likely(!(tmp & RCU_FGP_NEST_MASK)))
> > + *uarp = rcu_fgp_ctr; /* Outermost rcu_read_lock(). */
> > + else
> > + *uarp = tmp + RCU_FGP_COUNT; /* Nested rcu_read_lock(). */
> > + barrier();
> > +}
> > +
> > +static inline void rcu_read_unlock_fgp(void)
> > +{
> > + barrier();
> > + __get_cpu_var(rcu_fgp_active_readers)--;
>
> Shouldn't it be rcu_fgp_active_readers - RCU_FGP_COUNT?
> Although it is 1 by definition, it is more clear when understanding
> what's going on here.

Excellent point, fixed!

Thanx, Paul